The present invention relates to a process for the production of oxygen by adsorption of a stream of atmospheric air employing a unit of VPSA type.
The production of oxygen from atmospheric air by units of PSA type has undergone a significant expansion in recent decades. The improvements have related to the adsorbents, the technology and the process itself.
As regards the adsorbents, the most efficient units now use, within one and the same adsorber, a first layer intended to halt the humidity of the air and at least partially carbon dioxide. It will generally concern activated alumina or doped activated alumina which favors the adsorption of CO2. In the event of particularly polluted air, a portion at least of the activated alumina can be replaced with more resistant silica gel.
Halting of the nitrogen is preferably carried out on a zeolite of the LiLSX type with optionally a prelayer of zeolite of X type for halting the residual CO2 and beginning to adsorb nitrogen. Several types of LiLSX, more or less exchanged with lithium, for example, and optimized as a function of the nitrogen partial pressure within the bed, can be used in successive layers. Preferably, in the zone saturated with nitrogen at the end of the production phase, an adsorbent with a high adsorption capacity will be favored, whereas, in the mass transfer zone, an adsorbent with a high Henry's constant will be favored, while taking into account the thermal effects corresponding to these choices.
The diameters (or equivalent diameters in the case of adsorbent in the rod form) are generally between 0.5 and 2 mm. The dimension selected is a function of the duration of the cycle involved and is a compromise between kinetics and pressure drops.
The process proposed here is based a priori on the use of the abovementioned adsorbents but is not limited to their use. In particular, in the case of units employing short cycle times, for example less than 15 seconds, or a large number of adsorbers, for example 6 or more, it may be necessary to employ structured adsorbents (parallel-passage contactors, monolithic contactors, and the like) in order to avoid risks of attrition or of fluidization and pressure drops which are excessively high.
There have been a great many technological advances. They have concerned the valves, which are now faster, more reliable, more leaktight with regard to the atmosphere, and the like, devices, air compressors and vacuum pumps specially adapted by the manufacturers to the operating parameters of oxygen production units, drive by variable speed motor, more precise, more efficient and faster instrumentation and control system. Various types of adsorbers are used according to the flow rates involved or the local economic conditions: cylindrical adsorber having a vertical axis sometimes used in parallel until an assembly which can range, for example, up to 8 similar units for higher flow rates (reference is then made to group or cluster) is formed, cylindrical adsorber having a horizontal axis, radial adsorber. Several systems for holding the adsorbent in place and preventing attrition or fluidization have been employed (excess weight with ceramic or steel beads, membrane, balloon, spring, and the like). It is also possible to place in this field the management of the thermal effects with control of the thermal capacity of the adsorbent beds by addition of inert materials, such as phase change materials (PCMs). These types of developments, given non-exhaustively, can be applied in the context of the invention without it being able to be regarded as an improvement on what is provided here.
The last main field of improvement is the process itself. The term “process” is understood here to mean both the linking together of the stages which an adsorber will follow during its operation, and the characteristics of each of these stages: duration, amount of gas transferred, pressure, temperature, and the like.
Generally, the term PSA denotes any process for the purification or separation of gas employing a cyclical variation in the pressure which the adsorbent experiences between a high pressure, referred to as adsorption pressure, and a low pressure, referred to as regeneration pressure. Thus, this generic designation of PSA is employed without distinction to denote the following cyclical processes, to which it is also commonplace to give more specific names, depending on the pressure levels employed or the time necessary for an adsorber to return to its starting point (cycle time):                VSA processes, in which the adsorption is carried out substantially at atmospheric pressure, preferably between 0.95 and 1.25 bar abs, and the desorption pressure is less than atmospheric pressure, typically from 50 to 400 mbar abs;        MPSA or VPSA processes, in which the adsorption is carried out at a high pressure greater than atmospheric pressure, typically between 1.5 and 6 bar abs, and the desorption is carried out at a low pressure lower than atmospheric pressure, generally between 200 and 600 mbar abs;        PSA processes properly speaking, in which the high pressure is substantially greater than atmospheric pressure, typically between 3 and 50 bar abs, and the low pressure is substantially equal to or greater than atmospheric pressure, generally between 1 and 9 bar abs;        RPSA (Rapid PSA) processes, for which the duration of the pressure cycle is typically less than a minute;        URPSA (Ultra Rapid PSA) processes, for which the duration of the pressure cycle is of the order of a maximum of a few seconds.        
It should be noted that these various designations are not standardized and that the limits are subject to variation according to the authors.
With the preceding definitions, the invention relates both to VSA processes and to VPSA processes. Currently, due to the cycle times used, it also concerns the RPSA process and possibly, in the future, the URPSA process. In order to simplify the text, we will confine ourselves from now on to the term VPSA in order to encompass the field of application of the invention as has just been defined.
Whatever the type of PSA, an adsorber will begin a period of adsorption until it is charged in the constituent or constituents to be halted at the high pressure and will then be regenerated by depressurization and extraction of the adsorbed compounds, before being restored, in practice repressurized, in order to again begin a new adsorption period. The adsorber has then carried out a “pressure cycle” and the very principle of the PSA process is to link together these cycles one after the other; thus a cyclical process is concerned. The time which an adsorbent takes to return to its initial state is known as cycle time. In principle, each adsorber follows the same cycle with an offset in time, which is known as phase time or more simply phase. The following relationship thus exists:
Phase time=cycle time/number of adsorbents, and it is seen that the number of phases is equal to the number of adsorbers.
There exist units comprising only a single adsorber, whereas units, such as, for example, PSA H2 units, frequently comprise from 10 to 16 adsorbers.
A cycle generally comprises periods of:                Production or Adsorption, during which the feed gas is introduced via one of the ends of the adsorber, the most adsorbable compounds are adsorbed preferentially and the gas enriched in the least adsorbable compounds (product gas) is extracted via the second end. The adsorption can be carried out at an increasing pressure, at a substantially constant pressure, indeed even at a slightly decreasing pressure;        Depressurization, during which the adsorber, which is no longer fed with feed gas, is discharged via at least one of its ends of a portion of the compounds present in the adsorbent and the free spaces. Taking as reference the direction of circulation of the fluid in the adsorption period, it is possible to define cocurrentwise, countercurrentwise or simultaneously co- and countercurrentwise depressurizations;        Elution or Purge, during which a gas enriched in the least adsorbable constituents (purge gas) circulates through the adsorbent bed in order to help in the desorption of the most adsorbable compounds. The Purge is generally carried out countercurrentwise;        Repressurization, during which the adsorber is at least partially repressurized before again starting an Adsorption period. The repressurization can be carried out countercurrentwise and/or cocurrentwise, with various streams (feed, production, streams internal to the unit);        Dead time, during which the adsorber remains in the same state. These dead times can form an integral part of the cycle, making possible the synchronization of stages between adsorbers, or form part of a stage which has finished before the time assigned. The valves can be closed or remain in this state according to the characteristics of the cycle.        
Depressurization and Repressurization can be carried out in different ways, in particular when the PSA unit comprises a plurality of adsorbers (or of vessels). This thus leads to individual stages being defined in order to more exactly describe the gas transfers which occur between adsorbers (or vessels) and with the external environment (low-pressure waste gas, product gas, feed circuits).
The number of adsorbers is relatively independent of the linking together chosen for the stages, that is to say of the cycle. The use of several adsorbers makes it possible to directly use a stream resulting from a first adsorber in a second adsorber if the stages in question are simultaneous. It thus makes it possible to avoid the use of intermediate vessels, to better take advantage of the pressure gradients. This can also make it possible to optimize the operation of the devices, to render the production continuous, and the like.
As will be seen, there exist, at least to date, VPSA units comprising 1, 2, 3 or 4 adsorbers. It is also possible to use, in parallel, 2—or more—units of this type by optionally making joint use of some items of equipment (air filter, mufflers, production vessels, and the like, connected).
Contrary to many processes, in the case of the production of oxygen, the starting material, that is to say atmospheric air, is free and the energy consumption of the unit is one of the dominating items in the cost of production of the oxygen. For this reason, the slightest saving with regard to specific energy, at an unchanging capital expenditure, is advantageous because it directly and substantially impacts the production costs.
This is reflected in the facts by the existence of a large number of cycles which often differ only by a slightly different management of the incoming or exiting streams or by a slightly different adaptation of the arrangement of the stages to the number of adsorbers used.
The use of increasingly effective simulation programs now makes it possible to explore and to compare a very large number of variants and the gradual increase in the flow rates, by reducing the relative cost of the capital expenditure, makes possible, at a reasonable cost, a greater complexity in the management of the streams.
In the same way, the improvement in the kinetics of transfer of material or of heat related either to progress with regard to the adsorbents (increase in the intrinsic kinetics) or to the possibility of using smaller particles related to developments relating to adsorbers (radial adsorber, for example, monolithic adsorber, and the like) makes it possible to shorten the duration of the cycles and consequently the size of the adsorbers.
For all these reasons (free starting material, reduced influence of the capital expenditure), the energy consumption is increasingly assuming a dominating importance.
Starting from this, a problem which is posed is that of providing an improved process exhibiting an energetically high-performance cycle.